Co-Production of Electricity and Methanol in IGCC-Based Plants to Compensate Wind Power

Air and oxy-fuel reference cases in the DTU 30 kW swirl burner

Biofuels GS 2

Analytical techniques in combustion

Copenhagen, Danmark 2009

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Swirl burner

Table of Contents

1Introduction

2Aim

3Experimental methodology

3.1Description of the set-up

3.1.1Safety considerations

4Experiments and result

4.1False air/Air ingress

4.2Emissions of SO2 and NO

4.3Temperature measurements

4.4The loss on ignition analysis

4.5Mean values and variance for gas phase compositions

4.6Ash sampling

4.7Tuning of the burner in oxy fuel mode

4.8Scanning electron microscopy SEM

5Discussions and conclusions

6Appendix

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Swirl burner

1 Introduction

This report is a part of the Biofuels GS-2 Nordic course Analytical Techniques in Combustion, Part II at the Technical University of Denmark, DTU at the CHEC research group from February 2-6th 2009.

The combustion experiments are performed at the swirl-flow burner set-up (045-08) supervised by Maja Bøg Toftegaard. The group consists of

Daniel Fleig, Chalmers University of Technology, CTU

Stefan Hjärtstam, CTU

Daniel Kühnemuth, CTU

Frederik Lind, CTU

Linda Nørskov, DTU

Today, the anthropogenic emission of carbon dioxide is considered to be a major environmental issue. A high concentration of CO2 in the atmosphere contributes to global warming and oxy-fuel combustion is one of the emerging carbon capture and storage (CCS) technologies that could reduce CO2 in the atmosphere.

In oxy fuel combustion the fuel is burnt in oxygen instead of air. Recycled flue gas, mainly consisting of CO2, is used to cool the flame down to air-fired temperatures. The CO2 enriched flue gas can be cleaned and stored in deep porous sand structures or be used for enhanced oil recovery.

2 Aim

The purpose of the experiments is to evaluate fuel burnout, emissions of NO and SO2, axial temperature distribution and ash quality. Moreover, the goal is to establish a reference case for coal combustion in both oxy-fuel conditions and for combustion in air.

3 Experimental methodology

3.1 Description of the set-up

The experimental setup is a 30 kW down-fired swirl-flow burner in an alumina-lined vertical reactor operating with a maximum temperature of 1300oC and at a pressure of ~ -5 mbarg. The burner runs on natural gas, pulverized coal, straw, wood solo or in co-combustion operation (solid fuel particle size 20-500 µm).

Schematics of the reactor and the burner are given in figures 1 and 2, respectively. The inner diameter of the reactor is 31.5 cm and the total inner length is 193 cm. The reactor has a solid fuel feed system, 8 measuring ports along the side of the reactor, an ash sampling system (bottom ash, cyclone ash, and filter ash can be collected), temperature and gas analysers, and a data acquisition system.

Table 1: Geometric parameters.

3.1.1 Safety considerations

During natural gas combustion a UV flame guard monitors the emitted light from the flame. The system makes sure that the fuel supply is switched off in the case that the flame is extinguished, to prevent fuel accumulation and risk of explosion. When firing coal the particles shield the UV light and the flame guard cannot detect the flame, and to prevent false shut-downs the UV-detector is bypassed. Therefore, the combustion process must be monitored manually and the set-up cannot be left unattended when firing solid fuel.

Most of the hot surfaces on the reactor are water/air cooled or screened of, but there are some exceptions. Users are instructed to be aware, and warning signs are posted. Hot metal surfaces can cause skin burns.

The solid fuel feeding system requires manual filling. Coal contains minerals and heavy metals that are carcinogenic. When firing coal, toxic minerals and heavy metals are accumulated in the ash. Handling coal and ash requires wearing safety equipment; gloves, mask, goggles and laboratory coat and should preferable take place in a fume cupboard. The dust should not be breathed or come in contact with eyes. The chemical risk assessment (APV) should be read before handling the materials.

During oxy-fuel combustion gas bottles with oxygen (99,5 vol %) is used. Pure oxygen presents a potential fire hazard in contact with flammable materials. It is important to make sure that the system is completely sealed and that the gas bottles only are opened when correctly connected to the mixing gas panel.

4 Experiments and result

The test set-up is heated with natural gas combustion for 11 hours before initiating coal combustion.

The test conditions for the reference case of coal combustion in air are given in table 2. The solid fuel is a bitumen coal, El Cerrejon COCERR from Columbia, the chemical composition is given in the Appendix 1.

Table 2: Conditions at coal combustion in air

The conditions for oxy-fuel combustion of coal are given in table 3. The oxidizer consists of 30% oxygen in CO2.

Table 3: Conditions at oxy-fuel combustion of coal

4.1 False air/Air ingress

The expected O2 concentration in the flue gas is 4,39% in air combustion and 7,29% in oxy-fuel combustion but the measured average in O2 concentration in the flue gas was quite higher (6,95% in air combustion and 9,75 in oxy-fuel). This yields from incomplete combustion and air ingress somewhere in the system. With the assumption that the combustion is complete and the air ingress is in the combustion chamber the maximal amount of false air is 107 NL/min (18%) for the air fired case and 71 NL/min (16%) for the oxy-fuel case. But probably the air leakage is in the gas sample line and then the amount of air ingress is much smaller.

4.2 Emissions of SO2 and NO

The following table 4 shows the emissions of SO2 and NO. The emissions of NO in mg/ MJ are lower in the oxy-fuel case compared to the air-fired case. Less SO2 is also emitted during oxy fuel firing but the difference is negligible.

Table 4: Emissions of SO2 and NO

4.3 Temperature measurements

The combustion chamber is equipped with 8 measurement ports, named port 1 at the top to port 8 at the bottom of the furnace, according to figure 1.

Temperature profile is measured for an air fired case and an oxy-fuel case with 30 mol% O2 by using a thermocouple of S-type covered with a ceramic shield. On each measurement level, the temperature was measured 3 cm from the wall. This corresponds to a distance of 12.25 cm from the centre line. In each measurement port the temperature was logged for at least 10 to 15 minutes in order to achieve stabilized conditions. Once the temperature was stabilized, a time average was calculated.

The retrieved temperatures profiles for the air fired and the oxy-fuel case are shown in Figure 3. The temperature is described as a function of the distance from the burner outlet.

Figure 3: Temperature profile in air fired and oxy-fuel (30mol% O2) case

It can be stated that the temperatures in the two cases do not differ significantly. The biggest deviation between them is 24 K in port 1. As the temperature was measured just in 3 cm distance from the wall, it is assumed to be almost the wall temperature. In all ports except port 1, the temperature is slightly higher in the oxy-fuel case. If one has the aim to compare the air fired and the oxy-fuel case on bases of the same combustion temperatures, the oxygen concentration might have to be lowered.

One interesting exception of these overall trend is the temperature in the highest port, port 1, where the temperature is higher in the air fired case. These phenomena could maybe explained by the different flow characteristics of the two cases. The volume flow is with 427 mN3 in the oxy-fuel case about 1/3 smaller than in the air fired case with 600 mN3. Thereby the gas velocities in the combustion chamber are lower. At the top of the combustion chamber, at the height of port 1, the flame might not reach as close to the wall as in the air fired case.

4.4 The loss on ignition analysis

The loss on ignition (LOI) experimental procedure is performed in accordance with the Danish Standard DS 204.

The LOI is an expression of the weight difference of a powder sample between the dried condition, 105oC, 1hr and after being heated to 550oC, 2hr.

The LOI is performed on five ash samples, bottom ash and filter ash, for air combustion and oxy-fuel combustion, respectively. For oxy-fuel bottom ash two samples are analysed.

Firstly, the crucibles are dried (105oC, 1hr), then approximately 1g of ash is weighed into the crucible. The ash is dried (105oC, 1hr), and the dry weight is found. The crucibles are heated to 550oC, 2hr and the samples are weighed.

The LOI is calculated as

where LOI is % weight loss of the dried solid sample, m1 is the mass of the dried empty crucible, m2 is the mass of the crucible and dried sample, m3 is the mass of the crucible and sample after the high temperature heat treatment.

The moisture content in % of the dried sample, F, is calculated as

where m4 is the mass of the crucible and the moist sample. The results of the LOI analysis is given in table 5.

Table 5. Loss of ignition analysis

4.5 Mean values and variance for gas phase compositions

The average concentrations of the measured gases in the flue gas are listed in Table 6. CO2 for the oxy fuel case was not measured, since no instrument that could handle that high concentration was available. Further the standard deviation is listed to get knowledge about the variance.

Table 6: Average gas concentrations in the flue gas.

Figure 4 shows the variance in concentration for O2 and figure 5 the variance of CO2 in the flue gas for coal combustion with air as oxidiser.

Figure 4 O2 concentration in the flue gas with air coal combustion.

Figure 5: CO2 concentration in the flue gas with air coal combustion.

In figure 6 a comparison is made between the variance in concentration for the CO2 and O2. It is obvious that a high O2 concentration is coupled to a low CO2 concentration and vice verse. The fact that the graphs are more or less each other’s mirrors gives confidence in the gas analyse system. The sum of both concentrations should be a more or less a horizontal line.

Figure 6 Comparison of CO2 and O2 concentration in the flue gas with air coal combustion.

The following figures (figure 7, 8 and 9) shows the concentration of CO, NO and SO2 in the flue gas (coal combustion with air as oxidiser).

Figure 7 CO concentration in the flue gas with air coal combustion.

Figure 8 NO concentration in the flue gas with air coal combustion.

Figure 9 SO2 concentration in the flue gas with air coal combustion.

In figure 10 the CO, NO, SO2 and O2 concentrations are compared. It is seen that the O2, NO, SO2 concentrations are correlated. A reduction in O2 concentration is followed by a simultaneous reduction in NO and an increase in SO2 concentration. No plausible explanation has been found to describe this behaviour.

Figure 10 Comparison of CO, NO, SO2 and O2 concentrations in the flue gas with air coal combustion.

The following figures (figure 11, 12, 13, and 14) shows the concentration of O2, CO, NO and SO2 in the flue gas with oxy-fuel combustion of coal.

Figure 11 O2 concentration in the flue gas with oxy-fuel coal combustion.

Figure 12 CO concentration in the flue gas with oxy-fuel coal combustion.

Figure 13 NO concentration in the flue gas with oxy-fuel coal combustion.

Figure 14 SO2 concentration in the flue gas with oxy-fuel coal combustion.

4.6 Ash sampling

A particle sampling probe, shown in figure 15, is used for collecting the ash samples. Water with a temperature of ~60°C is used to cool the probe. The temperature of the cooling water is set to avoid condensation of flue gas on the probe surface. The probe is located in the bottom of the cylindrical furnace.

Figure 15. The ash sampling probe.

The sampling probe is funnel shaped and the sampling diameter is 150 mm. Comparing the sampling area with the horizontal cross section area yields a ratio of 1502/3152=0.23. A volume corresponding to 23 % out of the total flow would give isokinetic conditions, that is the same velocity in the reactor as in the sampling area. After the flue gas has passed the sampling probe the flue gas passes a container where bottom ash is collected. After the bottom ash collector there is a cyclone collecting ash and finally a filter, made of sintered stainless steel, which collects the last part of the fly ash. The particle sample system is shown in figure 16.

Figure 16. The particle sample system.

During the air-fired experiments the calculated total dry flue gas flow was 581 Nl/min. Through the ash probe a flue gas flow (calculated dry) of 319 Nl/min was sucked out. The isokinetic assumption, that 23 % of the total flow is to be sucked through the ash probe, was thereby not valid. Instead 55 % of the total flow was passing through the particle sampling probe. Assuming an evenly distributed fly ash in the flue gas, the theoretical mass flow of ash that was sucked out was 0.208 kg/h during the sampling period. The actual collected mass flow of ash was 0.103 kg/h (~0.056 kg bottom ash, ~0.002 kg cyclone ash and ~0.231 kg filter ash). If the 23% of the flue gas would be passing the sampling system the theoretical mass flow of ash is 0.088 kg/h.

The calculated total dry flue gas flow for the oxy fuel combustion case was 407 Nl/min. A flue gas flow (calculated dry) of 180 Nl/min was extracted by the ash probe. Thus, approximately 44 % of the total flow was passing through the particle sampling probe and flow condition was far from isokinetic. Assuming an evenly distributed fly ash in the flue gas, the theoretical mass flow of ash that was sucked out during oxy fuel combustion is the same as for the air-fired case since the fuel flow is the same for both cases. The measured mass flow of ash was 0.056 kg/h (~0.016 kg bottom ash, ~0.007 kg cyclone ash and ~0.129 kg filter ash). If the 23% of the flue gas passes the sampling system the theoretical mass flow of ash is 0.088 kg/h, which is the same as for air-fired conditions.

Regarding the accuracy of the ash sampling it should be mentioned that the bund, the cyclone and the filter were perhaps not cleaned enough before the sampling of the air-fired case. Possibly, there were some ash from previous experiments located in the sampling equipment. Before the oxy fuel experiments the sampling devices were cleaned properly.

4.7 Tuning of the burner in oxy fuel mode

During coal-firing the reactor pressure tends to oscillate, which creates an unstable pulsating flame. This pressure fluctuation influences the gravimetric fuel feeder as well. A test was carried out were the burner was tuned so that the primary oxidizer flow was reduced and the secondary oxidizer flow was increased. This change was motivated by the fact that the flame seemed to be blown of the burner outlet and thereby not attached to the burner. Initially (before 13:37) the primary flow was 90 l/min and the secondary flow was 410 l/min. After the change the primary and secondary flows were 70 l/min and 430 l/min, respectively. Once the new conditions were stabilized, the flame was actually more attached to the burner and the pressure oscillations were somewhat reduced. To see if it was possible to stabilize the flame even further, the primary/secondary split was changed (13:45) to 60 l/min primary oxidizer and 440 l/min secondary oxidizer. This change did not have any major impact on the behaviour of the flame. The primary flow was not reduced further because of the risk of plugging of the coal lance. It should be kept in mind that the flow rates were taken directly from the variable area flow meters. No correction to Nl/min were taken into account.

4.8 Scanning electron microscopy SEM

In order to analyse the composition of the sampled filter ash, a SEM-analysis is performed. For this, the samples have been prepared by coating with a carbon layer of about 30 µm in order to create an electrical conductive surface for the electron beam of the SEM.